NEW YORK (AP) -- Photographs taken by a spacecraft orbiting Mars indicate that active volcanoes may still exist on the red planet, further eroding its image as a dead world and offering prime sites to prospect for signs of Martian life.

Images from the European Space Agency's Mars Express Orbiter indicate geologically recent volcanic activity in the summit craters of five Martian volcanoes, with some areas showing activity as recently as 4 million years ago. Though long in human terms, 4 million years amounts to the most recent .01 percent of Martian history -- a strong suggestion that the planet retains a capacity for volcanic activity.

"I suspect that as we get more spacecraft in orbit that it will increase the chances of seeing some kind of active eruption," said Jim Head, a professor of geological sciences at Brown University in Providence, Rhode Island. He is one of more than 40 scientists who contributed to an analysis of the images to be published in this week's issue of the British journal Nature.

In the last few years researchers have found abundant evidence of ice at the Martian surface and signs that water flowed there in the past -- most recently with the United States' twin robotic rovers still exploring Mars.

There have also been signs of recent volcanic activity. The latest work suggests that water could still bubble up in hydrothermal springs on some of the planet's spectacular volcanic peaks.

"This is of great interest to biologists," said Michael Carr, a planetary scientist in the Menlo Park, California, office of the U.S. Geological Survey.

In recent years researchers have discovered that hydrothermal environments on Earth are remarkably rich in life. Hydrothermal vents on the ocean floor and hot springs on land provide the nutrients and energy to sustain rich ecosystems. Some biologists even argue that life began in such places.

Martian hydrothermal systems would probably look much like the steaming pools and spouting geysers of Yellowstone National Park, Carr said. If such environments do exist on Mars, they would be a critical place for future missions to investigate.

The Mars Express orbiter has been taking three-dimensional images of the Martian surface since January. The European Space Agency expects that by the end of its mission late next year Mars Express will have photographed the entire planet to a resolution of 33 feet (10 meters).

The researchers determined the age of the Martian volcanic features by counting craters on the Martian surface. When a volcano erupts and spreads lava over the landscape, it creates a smooth surface that is gradually pocked with craters as the planet is bombarded over the millennia by asteroids, comets and meteors.

The rate of that bombardment has been established using a number of different methods. So by counting the number of craters in a patch of Martian terrain researchers can tell how recently it was resurfaced by volcanic activity.